In a prior art bending machine such as a press brake, a sheetlike workpiece is placed between the upper and lower dies and the movable ram is operated to lower the upper die or lift the lower die thereby pressing the workpiece between these dies to perform a desired bending operation. When such a bending machine is used to bend an elongate workpiece so as form a bending line extending in a direction parallel to the longitudinal direction of the workpiece, there arises the problem of so-called "boat form", that is, the workpiece being less deeply bended at the central area of the bending line than at both end areas, as shown in FIG. 8.
The following are the chief causes of "boat form".
(1) The workpiece varies in physical characteristics (e.g., sheet thickness) from position to position in the longitudinal direction of the workpiece.
(2) Since the movable ram is stressed at both ends, the machine itself is resiliently deformed. This leads to a non-uniform stress distribution in which stress varies from position to position in the bending line of the workpiece.
There have been proposed several correction mechanisms in an attempt to prevent the occurrence of boat form. Typical of these correction mechanisms are disclosed in the following publications.
(1) Japanese Patent Publication No. 60-17610 (1985)
By inserting wedge-like keys above the upper die or under the lower die, the upper die and lower die are made close to each other in the areas corresponding to the areas of the workpiece where imperfect bending is likely to occur.
(2) Japanese Utility Model Laid-Open Publication No. 6-54416 (1994)
There are provided at least three driver units for lifting and lowering the ram and a deflection detector for detecting the deflection amount of the ram. A value of deflection detected by the deflection detector is fed back to a controller to control the middriver unit.
(3) Japanese Patent Publication No. 3-53046 (1991)
Each end of the movable apron is provided with a hydraulic cylinder for lifting and lowering the movable apron. The fixed apron or movable apron is provided at almost the center with a hydraulic cylinder for correcting the deflection of the apron. Based on the pressure of the lifting hydraulic cylinder and on the length of the workpiece, the amount of pressure oil to be supplied to the deflection-correcting hydraulic cylinder is controlled.
The first correction method (1) was found to be unsuitable for bending particularly an elongate workpiece because it has difficulty in carrying out bend angle detection along the longitudinal direction of the workpiece. In addition, even if a correction amount for the bend angle at each detecting point can be obtained from the result of inspection, correction can be rarely done by one step but a sequence of steps (i.e., trial bending.fwdarw.inspection .fwdarw.correction) must be repeated. Moreover, such a sequence of steps need to be carried out whenever bending length or sheet thickness is changed, which entails prolonged set-up time and increased cost due to a waste of test workpieces etc.
The second correction method (2) is intended to correct the deflection of the ram alone, but does not obtain the accurate distance between the upper and lower dies which distance actually affects the accuracy of bending, so that this method cannot accommodate variations in the thickness of a workpiece.
The third correction method (3) is based on the assumption that the pressure required for bending is proportional to the deflection of the apron (ram) and is designed to control the amount of pressure oil supplied to the deflection-correcting hydraulic cylinder by a control signal obtained from arithmetic operation. This method, therefore, fails in accurately correcting boat form in accordance with variations in the thickness of a workpiece.
The present invention has been made to overcome the foregoing problems and one of the objects of the invention is therefore to provide a bending machine which is capable of performing accurate in-line bending free from boat form, without making trial bends.